A proposal to make grouping of items in an array easier.
const array = [1, 2, 3, 4, 5];
// group groups items by arbitrary key.
// In this case, we're grouping by even/odd keys
array.group((num, index, array) => {
return num % 2 === 0 ? 'even': 'odd';
});
// => { odd: [1, 3, 5], even: [2, 4] }
// groupToMap returns items in a Map, and is useful for grouping using
// an object key.
const odd = { odd: true };
const even = { even: true };
array.groupToMap((num, index, array) => {
return num % 2 === 0 ? even: odd;
});
// => Map { {odd: true}: [1, 3, 5], {even: true}: [2, 4] }- Justin Ridgewell (@jridgewell)
- Jordan Harband (@ljharb)
Current Stage: 3, but there are webcompat issues that necessitate a change
Array grouping is an extremely common operation, best exemplified by
SQL's GROUP BY clause and MapReduce programming (which is better
thought of map-group-reduce). The ability to combine like data into
groups allows developers to compute higher order datasets, like the
average age of a cohort or daily LCP values for a webpage.
Two methods are offered, group and groupToMap. The first returns a
null-prototype object, which allows ergonomic destructuring and prevents
accidental collisions with global Object properties. The second returns
a regular Map instance, which allows grouping on complex key types
(imagine a compound key or tuple).
We've found a web compatibility issue with the name
groupBy. The Sugar library until v1.4.0 conditionally
monkey-patches Array.prototype with an incompatible method. By
providing a native groupBy, these versions of Sugar would fail to
install their implementation, and any sites that depend on their
behavior would break. We've found some 660 origins that use these
versions of the Sugar library.
In a similar situation, we renamed the Array.prototype.flatten
proposal to flat. Note that we did not rename it to flattened
(though it was considered). We've taken the same approach, renaming
groupBy to group.
- A polyfill is available in the core-js library. You can find it in the ECMAScript proposals section.
- Lodash